Do NOT delete your connection and recreate it as this will delete references to your original connection from your report and you will no longer be able to use your report. Further this type of action is not necessary to do in any situation.
You need to determine which object is causing you the issue and verify if your failing object is actually valid by viewing it in Sense Hub. If you cannot open the object in the Qlik Sense hub, it will never preview correctly in NPrinting.
It's not clear which version of NPrinting that you are using. To ensure you have the best NP logging possible, ensure that you have upgraded to at least the Sept. 2018 version of NPrinting or higher. The logs in c :\program data\NPrinting" will provide much better information about the issue as well.
You may need to submit a support request to investigate this further is fixing the problematic chart or filter does not resolve the issue.
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